1. Field of the Invention
This invention relates to a two-cycle engine of the spark ignition type, in which a mixture in a combustion chamber can be compressed and self-ignited at least in a low load operation region to effect activated heat atmosphere combustion. The mixture in the combustion chamber can be ignited, in a transition operation region between the activated heat atmosphere operation region and an ordinary spark ignition combustion operation region, by spark ignition at a timing earlier than the spark ignition timing in the ordinary spark ignition combustion operation region to stabilize the combustion condition to eliminate abnormal combustion noise or harmful effects of heat.
2 . Description of Background Art
In a two-cycle engine of the spark ignition type having a carburetor, fresh air is obtained and mixed with fuel with air in advance being supplied into a combustion chamber. An exhaust port and a scavenging port which are opened and closed by a piston are formed on an inner circumferential face of a cylinder bore. The mixture, pressurized in a crank chamber in advance, is supplied into a cylinder chamber through the scavenging port while burnt gas in the cylinder chamber is exhausted through the exhaust port, and the mixture compressed in the cylinder chamber is ignited by means of an ignition plug.
In such a conventional two-cycle engine of the spark ignition type as described above, if the exhaust port is made large in order to set the output power and the efficiency in a high speed, high load operation region to a level higher than a high level, then, in a low load operation region, the amount of unburnt hydrocarbons in the exhaust gas is increased by blow-by of fresh air or unstable combustion, resulting in deterioration of the fuel cost.
In order to eliminate the above problems, the present inventor has developed and applied for a patent for an engine wherein an exhaust control valve such as an exhaust air passage opening ratio adjustment means is actuated to an exhaust air passage opening ratio in response to the engine speed and the throttle valve opening to control the in-cylinder pressure when the exhaust opening is closed by the piston appropriately at least in a low load operation region so that the mixture in a combustion chamber can be activated by heat energy of burnt gas remaining in the combustion chamber to cause the mixture in the combustion chamber to be compressed and self-ignited at an ignition timing preferable for operation of the engine (Japanese Patent Application No. Heisei 5-187488).
The combustion wherein the ignition timing preferable for operation of an engine is controlled positively to cause activated heat atmosphere combustion to take place in this manner is hereinafter referred to as AR combustion, and the operation region in which such AR combustion takes place will be hereinafter referred to as AR operation region.
In such a two-cycle engine of the spark ignition type in which AR combustion can take place as described above, an ignition plug generates a spark at a predetermined ignition timing in whatever operation condition of the engine. Since the amount of fresh air taken in is small in an AR combustion operation which makes use of heat energy of burnt gas as shown in FIG. 6, when the engine speed Ne rpm) is higher than a very low speed, the output power is low compared with that in an ordinary combustion operation wherein ignition firing takes place, but in a transition operation region A between the ordinary combustion operation region and the AR combustion operation region, ordinary combustion and AR combustion take place in a mixed manner.
In the AR combustion operation region, if the exhaust air passage opening ratio is set appropriately in response to the comparatively low engine speed and the comparatively small throttle valve opening by way of the exhaust control valve, then compression self-ignition in which the ignition timing is stabilized takes place without being influenced by the spark ignition timing by the ignition plug. However, in the transition operation region A in which ordinary combustion and AR combustion take place in a mixed condition as described above, in a certain cycle, fresh air activated by heat energy of burnt gas is ignited by compression self-ignition at an early timing to cause a sudden combustion reaction so that, as shown in FIGS. 8B and 8C, a great pressure variation and a high indicator pressure are provided. In a next cycle, the ignition timing is delayed by a drop in expansion end temperature due to the early ignition, and a small pressure variation and a low indicator pressure are provided. Such phenomena take place alternately, and the ignition timing is not stabilized and high exhaust noise is produced.
As set forth in Japanese Patent Laid-Open Application No. Showa 53-109007, an operation method for a spark ignition engine includes a cylinder which is normally operated with spark ignition and operated upon a particular operation such that both of an intake air flow and an exhaust air flow or only the exhaust air flow is restricted so as to effect operation with compression ignition, and upon transition to the compression ignition operation, said cylinder is operated with spark ignition at a compulsorily advanced ignition timing. This operation takes place as if the ignition timing was advanced in a transition operation region between an ordinary combustion operation region and an AR combustion operation region within a single cylinder. However, since, upon transition to the compression self-ignition operation (different from the AR combustion operation) in which the compression self-ignition timing is not controlled, the exhaust air flow is restricted so as to effect a compression self-ignition operation with one of two cylinders while only the ignition timing is advanced without restricting the exhaust air flow with the other cylinder, combustion is started prior to the top dead center to cause an excessively earlier ignition condition. Consequently, occurrence of harmful effects of heat or abnormal combustion noise is invited.